1. Field of the Invention
The present invention relates to a method for conducting electric activation of electric double layer capacitors in which nonporous carbonaceous electrodes are soaked in an organic electrolytic solution, and particularly to a method for conducting electric activation of electric double layer capacitors which show a behavior such that the electrostatic capacitance increases as the capacitors experience a voltage.
2. Description of the Related Art
Capacitors can repeat charging and discharging with a big electric current and, therefore, are promising as devices for electric power storage with high charging-and discharging frequency. Capacitors are, therefore, hoped to be improved in energy density, rapid charging/discharging characteristics, durability, etc.
The fact that carbonaceous electrodes are soaked in an organic electrolytic solution to form electric double layer capacitors is known. Michio Okamura “Electric Double-layer Capacitors and Power Storage Systems” 2nd Edition, The Nikkan Kogyo Shimbun, Ltd., 2001, pages 34 to 37 discloses an electric double layer capacitor comprising a bath partitioned into two sections with a separator, an organic electrolytic solution filled in the bath and two carbonaceous electrodes, one electrode being soaked in one section of the bath and the other electrode being soaked in the other section of the bath. The organic electrolytic solution is a solution containing a solute dissolved in an organic solvent. Tetraethylammonium tetrafluoroborate (Et4NBF4) or the like are disclosed as solutes and propylene carbonate is disclosed as a solvent. As the carbonaceous electrodes, activated carbon is employed. The activated carbon refers to shapeless carbon which has a veil, large specific surface area because it has innumerable fine pores. In the present specification, shapeless carbon having a specific surface area of about 1000 m2/g or more is referred to as activated carbon.
Japanese Patent Laid-open Publication No. H11(1999)-317333 discloses a nonporous carbonaceous material as carbonaceous electrodes for use in electric double layer capacitors. The carbonaceous material comprises fine crystalline carbon similar to graphite and has a specific surface area of 300 m2/g or less, which is smaller than that of activated carbon. Nonporous carbonaceous electrodes generate electrostatic capacitance in a mechanism completely (different from that of carbonaceous electrodes made of activated carbon. It is believed that application of voltage makes electrolyte ions intercalated with solvent between layers of graphite-like fine crystalline carbon, resulting in formation of an electric double layer.
Japanese Patent Laid-open Publication No. 2002-25867 discloses production of carbonaceous electrodes using needle coke or infusibilized pitch as a raw material. The needle cole refers to easily graphitizable calcined coke with well-developed needle crystals. Needle coke has a very high electrical conductivity and a very low coefficient of thermal expansion, and also has a high anisotropy based on its graphite crystal structure. Generally, needle coke is produced by a delayed coking process using specially-treated coal tax pitch or petroleum-derived heavy oil as a raw material.
Japanese Patent Laid-open Publication No. 2000-77273 discloses an electric double layer capacitor including nonporous carbonaceous electrodes soaked in an organic electrolytic solution. The organic electrolytic solution must have ion conductivity, and therefore the solute is a salt composed of a cation and an anion combined together.
The cations disclosed include lower aliphatic quaternary ammonium such as tetraethylammonium, tetrabutylammonium and triethylmethylammonium; lower aliphatic quaternary phosphonium such as tetraethylphosphonium; and imidazolium derivatives. As the anion, tetrafluoroboric acid, hexafluorophosphoric acid and the like are disclosed. The solvent of the organic electrolytic solution is a polar aprotic organic solvent. Specifically, ethylene carbonate, propylene carbonate, γ-butyrolactone, sulfolane and the like are disclosed.
An electric double layer capacitor including nonporous carbonaceous electrodes soaked in an organic electrolytic solution exhibits a charging/discharging characteristics different from that of capacitors using activated carbon as carbonaceous electrodes. The charging/discharging characteristics is described in Michio Okamura “Electric Double-layer Capacitors and Power Storage Systems” 2nd Edition, The Nikkan Kogyo Shimbun, Ltd. 2001, pages 77 to 81. FIG. 1 is a graph showing one example of the behavior in which voltage changes with time when charging and discharging at a constant current are applied repeatedly to that type of the electric double layer capacitors (quoted from Michio Okamura “Electric Double-layer Capacitors and Power Storage Systems” 2nd Edition, The Nikkan Kogyo Shimbun, Ltd., 2001, page 81, FIG. 3–15).
In the charging/discharging curve of FIG. 1, the voltage stands up within a short period of time in the first constant current charging started from 0 V, and the voltage increment becomes slow at around 2.2 V. In other words, the slope (dv/dt) is substantially constant before around 2.2 V, decreases quickly at around 2.2 V, and again is substantially constant after around 2.2 V.
In a constant current charging curve, an electrostatic capacitance corresponds to the slope of the curve. High slope values mean low electrostatic capacitances, and low slope values mean high electrostatic capacitances. So the initial charging curve of FIG. 1 shows that the electrostatic capacitance is small in an early stage of charging and that an electrostatic capacitance substantially starts to develop at around 2.2 V.
On the other hand, in the second and subsequent charging processes, the voltage increases monotonously and a constant electrostatic capacitance is obtained from the early stage of charging like conventional activated carbon electrodes. In other words, when this type of the electric double layer capacitors are first allowed to experience a voltage, the electrostatic capacitance of the capacitor increases and a large capacitance is successfully obtained.
Japanese Patent Laid-open Publication No. 2000-277397 and Japanese Patent Laid-open Publication No. 2001-223143 disclose methods for conducting electric activation of electric double layer capacitors. “Electric activation” means a first charging treatment of an electric double layer capacitor, namely, a process in which a polarizing electrode is first allowed to experience a voltage. This expression is used because the first charging treatment is considered to serve as activation by which an electrostatic capacitance is developed. These documents disclose that the electric capacitance of an electric double layer capacitor is increased through optimization of conditions for electric activation.
FIG. 2 is a graph showing one example of a charging/discharging curve of an electric double layer capacitor in a conventional electric activation method. In this method, an electric double layer capacitor is first subjected to a constant current charging until the interelectrode voltage reaches a predetermined voltage which is not lower than the rated voltage but is not higher than the decomposition voltage of the electrolytic solution, and then a constant voltage charging is conducted for a certain period of time. After a constant current discharging is conducted, charging/discharging is conducted under the rated current and the rated voltage.
However, for practical use as an auxiliary power source of electromobiles, batteries and power plants, electric double layer capacitors are demanded to have increased performances.